Light emitting diodes (LEDs) are semiconductor devices that convert electric energy to light. Inorganic LEDs typically include an active layer of semiconductor material and a P-N junction formed at an interface between two oppositely doped layers. When a bias is applied across the P-N junction, holes and/or electrons are injected by the P-N junction into the active layer. Recombination of holes and electrons in the active layer generates light that can be emitted from the LED. The structure of the device, and the material from which it is constructed, determine the intensity and wavelength of light emitted by the device. Recent advances in LED technology have resulted in highly efficient solid-state light sources that surpass the efficiency of incandescent and halogen light sources, providing light with equal or greater brightness in relation to input power.
Conventional LEDs generate narrow bandwidth, essentially monochromatic light. However, it may be highly desirable to generate wide bandwidth, polychromatic light, such as white light, using a solid state light source. One way to produce white light from conventional LEDs is to surround a single-color LED chip or die with a light conversion material, such as a phosphor. In general, phosphors absorb light having shorter wavelengths and re-emit light having longer wavelengths. At least some of the light emitted by the LED chip at a first wavelength (primary light) may be absorbed by the phosphor, which may responsively emit light at a second wavelength (secondary light). The primary light emitted by the LED chip and the secondary light emitted by the phosphor particles may combine to produce light having a plurality of wavelengths, which may be perceived as having a different color than either the primary light or the secondary light.
For example, light from a blue-emitting LED chip has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, such as cerium-doped yttrium aluminum garnet (YAG:Ce). The phosphor material absorbs and “downconverts” some of the blue light generated by the LED chip. That is, the phosphor material generates light, such as yellow light, in response to absorbing the blue light. Thus, some of the blue light generated by the LED chip appears to be converted to yellow light. Some of the blue light from the LED chip passes through the phosphor without being changed, however. Accordingly, the overall LED/phosphor structure or package emits both blue and yellow light, which combine to provide light that is perceived as white light.
FIG. 1 is a region of a 1931 International Commission on Illumination (CIE) chromaticity diagram illustrating the color point distribution of packaged LEDs configured to emit white light fabricated according to conventional methods. Referring now to FIG. 1, a plurality of blue LED chips are configured to emit light over a wavelength range of 451 to 469 nanometers (nm). A yellow-emitting phosphor is deposited on all of the blue LED chips according to conventional methods such that the overall LED/phosphor package emits both blue and yellow light, which combine to provide light that is perceived as white light. Lines 101, 102, 103, 104, and 105 respectively represent the color points of light that may be produced by the blue LED chips having emission wavelengths of 451 nm, 455 nm, 461 nm, 465 nm, and 469 nm in combination with the yellow-emitting phosphor. As shown in FIG. 1, the lines 102, 103, and 104 fall inside a 7-step Mac Adam ellipse 115 around a targeted color point 120 at the center of the ellipse 115. The MacAdam ellipse 115 is an elliptical region of chromaticity coordinates that is defined based on a center, a tilt angle relative to a horizontal axis, and a level of variance. The color points contained within the MacAdam ellipse 115 are indistinguishable to the human eye from the targeted color point 120 at the center of the ellipse 115. However, not all of the packaged LEDs emit white light having a color point within the ellipse 115. In particular, the LED chips emitting light at wavelengths of 451 nm and 469 nm, respectively represented by lines 101 and 105, fall outside of the 7-step Mac Adam ellipse 115.
In some instances, it may be advantageous for packaged LEDs targeting a specific color point to emit white light within a 4-step Mac Adam ellipse around the targeted color point. A tighter distribution may result in a higher yield, so targeting the color point within a 4 step MacAdam ellipse may tighten the distribution and thus improve yields. However, as illustrated in FIG. 1, the electrical and/or optical parameters of individual packaged LEDs that are fabricated according to conventional methods may vary considerably, for example, due to routine process variations. Accordingly, improved fabrication methods may be desired.